The known VoIP (Voice over IP) communication technology allows IP (Internet Protocol) telephone communications between two IP telephones over the Internet.
Japanese Unexamined Patent Publication JP 2003-69602-A describes an IP telephone system. In this system, an IP telephone client A sends a call origination control packet containing an IP telephone address of an IP telephone client B. Upon receipt of this packet, an IP telephone server A performs a procedure for holding an unfixed IP address of the client A until the end of the IP telephone communication, and sends the packet to an IP telephone server B on the client B side. Upon receipt of this packet, the server B performs a procedure for holding an unfixed IP address of the client B until the end of the IP telephone communication, and sends the packet to the client B. Upon receipt of this packet, the client B notifies a user of the incoming call in a predetermined form. If an instruction is provided by the user for responding to the incoming call, the client B sends, to the client A, a response packet for responding to the call origination control packet.
Japanese Unexamined Patent Publication JP 2003-283670-A describes a VoIP contact center system which integrates systems for audio and data. In this system, an audio service apparatus and a data service apparatus are connected to a switching system installed only in a main center, and received audio is converted into the IP format for integration with data to provide to a satellite center and a residential satellite center, to thereby provide a service for allowing a customer to listen to audio explanation while the customer and an operator watch the same Web screen.
The IP telephone performs call control via the call control and management server on the network. For the call control, SIP (Session Initiation Protocol), H.323 or the like is used as a communication protocol. For initiating a call, the IP telephone typically establishes a session with another IP telephone via the call control and management server having a SIP server function. After the session is established, the IP telephone performs P2P (peer-to-peer) audio packet exchange with the other IP telephone.